I am surprised nobody has started a topic about this yet.
Currently there is a bug in the game where RWR will sporadically fail to work on planes that are equipped with MAWS. As you can see in the following clip that there is no MSL warning on the Eurofighter in these 3 instances.
Here is bug report.
https://community.gaijin.net/issues/p/warthunder/i/0FCWYsUo1gof